Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide systems and methods for power converters with self-regulated power supplies. Merely by way of example, some embodiments of the invention have been applied to flyback power converters. But it would be recognized that the invention has a much broader range of applicability.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system with source switching. The power conversion system 100 (e.g., a power converter) includes a controller 102 (e.g., a pulse-width-modulation controller), a transistor 120 (e.g., a MOSFET), resistors 130 and 132, a primary winding 142, a secondary winding 144, an auxiliary winding 146, capacitors 150, 152, 154 and 156, a full wave rectifying bridge (e.g., BD) including diodes 160, 162, 164 and 166, and diodes 170 and 172. The controller 102 includes terminals 110, 112, 114, 116 and 118. As an example, the controller 102 is a chip, and the terminals 110, 112, 114, 116 and 118 are pins.
The terminal 112 is configured to receive a power supply voltage 180 (e.g., VDD). As shown in FIG. 1, an AC input voltage 182 is rectified by the full wave rectifying bridge (e.g., BD). The full wave rectifying bridge (e.g., BD), together with the capacitor 150, generates a voltage 184 (e.g., Vbulk). The voltage 184 is received by one terminal of the resistor 130, and the other terminal of the resistor 130 is connected to the terminal 116. Additionally, the terminal 116 is connected to one terminal of the capacitor 156, and the other terminal of the capacitor 156 is biased to the primary-side ground.
The resistor 130 and the capacitor 156 serve as parts of an RC circuit, and the RC circuit performs a charging function to raise a voltage 186 at the terminal 116. Within the controller 102, there is a voltage clamping circuit that sets the upper limit of the voltage 186. When the voltage 186 increases, the voltage drop from the terminal 116 to the terminal 118, which is equal to the voltage drop from the gate terminal of the transistor 120 to the source terminal of the transistor 120, becomes larger than a threshold voltage of the transistor 120. If the voltage drop from the gate terminal of the transistor 120 to the source terminal of the transistor 120 becomes larger than the threshold voltage of the transistor 120, the transistor 120 is turned on, acting as a source follower. When the transistor 120 is turned on, a switch within the controller 102 that controls the internal connection between the terminals 118 and 112 is closed, and the terminal 118 is connected to the terminal 112 internally through one or more components of the controller 102. If the switch within the controller 102 that controls the internal connection between the terminals 118 and 112 is closed, the controller 102 charges the capacitor 154 to raise the voltage 180.
When the voltage 180 becomes larger than a predetermined under-voltage-lockout threshold of the controller 102, the controller 102 opens the switch within the controller 102 so that the internal connection between the terminals 118 and 112 is disconnected. Also, if the voltage 180 becomes larger than the predetermined under-voltage-lockout threshold, the controller 102 uses the terminal 118 to turn on and off the transistor 120, and the voltage 180 is provided by the auxiliary winding 146 together with one or more other components. Additionally, the power conversion system 100 provides an output current 158 and an output voltage 159 to a load 156.
As shown in FIG. 1, the power conversion system 100 includes a simple structure that can provide fast start-up, so the power conversion system 100 often are used in certain chargers for cellular phones. But the power conversion system 100 also has its weaknesses. For example, the power conversion system 100 uses the auxiliary winding 146 to provide the voltage 180, but the auxiliary winding 146, as an extra component, can make the power conversion system more costly and less efficient.
FIG. 2 is a simplified diagram showing a conventional flyback power conversion system with source switching. The power conversion system 200 (e.g., a power converter) includes a controller 202 (e.g., a pulse-width-modulation controller), a transistor 220 (e.g., a MOSFET), a resistor 232, a primary winding 242, a secondary winding 244, capacitors 250, 252 and 254, a full wave rectifying bridge (e.g., BD) including diodes 260, 262, 264 and 266, and a diode 270. The controller 202 includes terminals 210, 212, 214, 216 and 218. As an example, the controller 202 is a chip, and the terminals 210, 212, 214, 216 and 218 are pins.
The terminal 212 is coupled to the resistor 230 and the capacitor 254, and is configured to receive a power supply voltage 280 (e.g., VDD). The terminal 216 is at a voltage 286, the terminal 218 is at a voltage 288, and the terminal 210 is at a voltage 290. If the transistor 220 is turned on, a current 292 flows through the transistor 220 to the terminal 218. Also, the transistor 220 includes a drain terminal 222, a gate terminal 224, and a source terminal 226. The terminal 216 is coupled to the gate terminal 224, and the terminal 218 is coupled to the source terminal 226.
As shown in FIG. 2, an AC input voltage 282 is rectified by the full wave rectifying bridge (e.g., BD). The full wave rectifying bridge (e.g., BD), together with the capacitor 250, generates a voltage 284 (e.g., Vbulk). The voltage 284 is received by one terminal of the resistor 230. The other terminal of the resistor 230 is connected to the terminal 212 of the controller 202 and also to one terminal of the capacitor 254. The other terminal of the capacitor 254 is biased to the primary-side ground. Additionally, the power conversion system 200 provides an output current 258 and an output voltage 259 to a load 256.
As shown in FIG. 2, the power conversion system 200 uses the resistor 230 to convert the voltage 284 to the voltage 280 and also to provide the voltage 280 to the terminal 212. Without using an auxiliary winding, the cost of the power conversion system is lowered. But the power conversion system 200 has its weaknesses. For example, the resistance of the resistor 230 needs to be small in order to limit the voltage drop between the voltage 284 and the voltage 280, but such small resistance often causes significant energy consumption by the resistor 230. As another example, when the power conversion system 200 operates under normal conditions, some energy is transmitted to the terminal 212 through oscillation rings in the voltage 288 of the terminal 218. With such transmitted energy, the power conversion system 200, under influence of certain parasitic components, sometimes cannot provide a stable magnitude for the voltage 280. Under some conditions, the power conversion system 200 cannot even provide sufficient energy to sustain a proper magnitude for the voltage 280.
Hence it is highly desirable to improve the techniques related to flyback power conversion system with source switching.